Lots of factors for me:
- The price. A 10-hour game being $10 is fine enough. A 2-hour game being $10 isn't.
- The pacing. I can't stand it when most of the game's length is due to frequent random encounters, lots of backtracking, dungeons that are longer than they should be, filler content such as pointless fetch quests, and puzzles that cause me to be stuck for half an hour to solve.
- The developer. Not everyone might admit this, but we all do it. If I know the developer makes good games, I'll be more likely to check out longer games by them, but an unknown developer or one whose games I did not enjoy, I'll most likely pass on their 30-hour epic. That's why I think it is better to start off making shorter games, making it easier for people to download them, to play them fully and to get to know you as a reliable developer, and only then partake in longer games.
- The nature of the game. I personally am biased towards linear, story-rich games, so I am unlikely to play a 30+ hour game that's entirely sandbox.
On the other hand, games that are too short might not seem worth your time, especially if they are commercial.
I personally feel that if you're going commercial with your RM game, make it 4-5 hours in length at the very least. There are a lot of free RM games out there, so charging people any amount of money for a 1-2 hour game just looks bad on you.